Noble metal nanostructures including gold, silver, and gold or silver alloy have been studied in various applications due to their plasmon resonance. The phenomenon of plasmon resonance has significant applications along with control over the nanostructures in terms of the size and morphology. Examples of such applications include colorimetric sensors, conversion to clean water by the decomposition of water using photocatalysts, conversion of carbon dioxide to hydrocarbon gases, and oxidation of organic contaminants. In photocatalysts, absorbing light and accelerating a photoreaction are the most significant parts of photocatalysis. Additionally, a photocatalyst ideally has a high absorption rate of sunlight or visible light, and has high reactivity, non-toxicity, photostability, and chemical inactivity. Typically, a photocatalyst is a solid semiconductor having an ability to create electron-hole pairs when irradiated by light.
Plasmonic nanoparticles of noble metals with high absorption coefficients in the visible light region can serve as an alternative to sensitizers improving the absorption of a semiconductor, and they are mainly influenced by size and morphology. Additionally, the obtained hybrid catalysts have resistibility against degradation during the photoreaction. Various studies on noble metal/carrier hybrid photocatalysts have been reported. For example, the studies have been conducted on Ag/TiO2/graphene, Au/TiO2, and Ag/ZnO. On the other hand, Ag/AgCl has high activity and enhanced stability as a metal/semiconductor hybrid photocatalyst due to its fast separation and transportation of photogenerated electron-hole pairs. Multiple methods have been established in order to synthesize Ag/AgCl hybrid materials. Herein, such methods include an ion exchange reaction between Ag+ and Cl− by UV or laser light, hydrothermal synthesis, and thermal decomposition in an ionic liquid, etc. However, most of these procedures are disadvantageous in that they require complicated reaction steps, long reaction times, high reaction temperatures, or have reduced control of the size and morphology of the Ag/AgCl structure due to a rapid reaction rate between the silver and chloride ions. Additionally, controlling the composition of Ag and AgCl in the hybrid material is the most challenging aspect not only to obtain a high absorbability of visible light but also to obtain a catalytic activity of Ag nanomaterials. Until these days, the maximum level of Ag in the Ag/AgCl hybrid structure has been limited to 80% in all previous reports.
Recently, TiO2-based photocatalysts have been intensively investigated upon the discovery of the water electrolysis effect. TiO2-based photocatalysts have a band gap that is wider than 3.2 eV. It enables TiO2-based photocatalysts to be chemically stabilized, but, at the same time, restricts the response of TiO2-based photocatalysts to the UV fraction of solar energy (about 4%) due to a low efficiency in visible and near infrared regions. In addition, the fast recombinant rate of photogenerated electron-hole pairs also decreases the catalytic efficiency of TiO2-based photocatalysts. Over the past decades, considerable efforts have been devoted in order to improve their visible light absorption coefficient, including ion doping, noble metal deposition, anchoring organic dye molecules on the surface of photocatalysts, etc. Although there has been some progress enhancing photocatalytic efficiency in visible light, the limited amount and easy leaching of dopants may adversely influence the activity and chemical stability of photocatalysts. Additionally, dye molecules often may be self-degraded in dye-sensitized photocatalysts. On the other hand, some research groups have synthesized novel visible light catalysts, such as Bi2WO6, CFe2O4/TaON, and Cu2(OH)PO4, in addition to TiO2-based photocatalysts. However, these photocatalysts still have some drawbacks such as low activity and poor stability, etc. Therefore, developing new efficient visible light photocatalysts with high activity, stability, and recyclability still remains as a significant challenge.
Meanwhile, over the past decades, considerable amounts of researches have been devoted to the rational design and construction of hybrid nanostructures in order to enhance photocatalytic performance by modulating the properties of individual constitutional elements. For example, heterostructures based on novel metal nanoparticles and semiconducting supporters have received great attention due to improved photocatalytic performances thereof which are derived from increased absorbance by an ability to create electron-hole pairs, and an effect of the surface plasmon (SPR).
In previous researches on the synthesis and photocatalytic applications of Ag/AgBr, researchers mainly devoted their attention towards Ag/AgCl hybrid structures. Although Ag/AgBr photocatalysts exhibit superior photocatalytic activity compared to Ag/AgCl, very few studies are available regarding the synthesis of well-defined Ag/AgBr nanostructures. In addition, the reported procedures typically require tedious reaction steps, high reaction temperatures, non-aqueous solvents, and low reagent concentrations. Wang et al. prepared a Ag/AgBr plasmonic photocatalyst using Ag2MoO4 (Highly Efficient Visible-Light Plasmonic Photocatalyst Ag@AgBr. Chem. Eur. J. 2009, 15, 1821-1824). However, the synthesis was conducted under high pressure and temperature (over 180° C.), and required at least a reaction time of 8 hours or more. Further, the obtained hybrid particles were agglomerated, and the size thereof was greater than 1 μm. Xiao et al. synthesized Ag/AgBr cubic cages in a non-aqueous phase using a sacrificial template process by reacting for 24 hours with silver precursors at a low concentration of 1 mM. (Cubic Cages with Efficient Visible-Light-Induced Photocatalytic Activity. Appl. Catal. B Environ. 2015, 163, 564-572). Kuai et al. prepared micro-sized Ag/AgBr in an aqueous phase by reacting with AgNO3 in the presence of hexadecyl trimethylammonium bromide (CTAB) and ammonia for 8 hours at 120° C. in an autoclave (Facile Subsequently Light-Induced Route to Highly Efficient and Stable Sunlight-Driven Ag—AgBr Plasmonic Photocatalyst. Langmuir. 2010, 26, 18723-18727).